To reduce the size of a resonator using a slot line, a design approach of forming the slot line in a stepped impedance structure is already known (for example, see “ANALYSIS, DESIGN AND APPLICATIONS OF FIN LINES”, Bharathi Bhat, Shiban K. Koul, PP. 316-317 published by ARTECH HOUSE, INC, U.S.A. 1987, and “MAIKUROHA KAIRO NO KISO TO OHYO (BASICS AND APPLICATIONS OF MICROWAVE CIRCUITS), Yoshihiro KONISHI, Sougou Denshi Shuppannsha, p. 169 issued in 1990 (first edition). By forming the widths of both ends of the slot line to be larger and by forming the width of the central portion to be narrower, the impedance of both ends of the slot line becomes inductive and the impedance of the central portion becomes capacitive so that the impedance is changed in this approach, in a stepwise manner along the length of the slot line. The length of the slot line required for obtaining the same resonant frequency can be reduced with this arrangement.
A typical example of the above-described stepped-impedance-structured slot resonator is shown in FIG. 13B, a top view illustrating a substrate forming the slot resonator, and FIG. 13A, a sectional view taken along line A-A in FIG. 13B. A conductive film 10 having conductor openings APa, APb, and APc is formed on the top surface of a dielectric substrate 1. The conductor openings APa, APb, and APc form one conductor opening formed, as a whole, in a dumbbell shape. The widths (which can also be referred to as “diameters” since the openings APa and APb in this example are circular) of the conductor openings APa and APb at both ends are formed relatively large and the width of the central conductor opening APc is relatively small. Accordingly, both ends of the slot are inductive, while the central portion of the slot is capacitive.
The broken lines in FIG. 13A schematically indicate magnetic lines of force of this slot resonator. The magnetic distribution of this resonator is represented by the magnetic lines of force. In the stepped-impedance-structured slot resonator, magnetic field vectors are directed upward in the inductive area at one end, while magnetic field vectors are directed downward in the other inductive area at the other end, and the overall slot resonator serves as a magnetic dipole. Magnetic field energy produced by the resonance operation mostly concentrates in the inductive areas formed by the conductor openings APa and APb, while electric field energy is mostly distributed in the capacitive area formed by the conductor opening APc. In this manner, the slot resonator serves as a lumped-constant circuit by separating the storage area of the magnetic field energy from the storage area of the electric field energy, thereby achieving the miniaturization of the slot resonator.
The size of the slot resonator is inversely proportional to the resonant frequency. Accordingly, forming a slot resonator into a stepped-impedance structure as described above is effective in reducing the size of the resonator when the resonant frequency is relatively low. Additionally, a larger impedance step ratio of the capacitive area to the inductive areas is more effective in reducing the size of the resonator.
In the example shown in FIG. 13B, therefore, it is effective if the line width of the conductor opening APc is formed to be narrow and if the line length is reduced. However, because of a restriction on the pattern forming precision of the conductive film, the line width cannot be formed to be extremely narrow. Additionally, a change in the capacitance of the capacitive area caused by dimensional variations in the line width becomes noticeable as the line width becomes narrower. Thus, it is more difficult to obtain a predetermined resonant frequency with high precision as the line width of the conductor opening APc in the capacitive area becomes narrower.
To solve the above-described problems, it is an object of the present invention to provide a resonator, a filter, and a communication apparatus that can be easily miniaturized even if the resonant frequency is relatively low.